Man hole covers for underground utilities, underground storage tanks, and sewer lines have generally been make from iron or steel to provide sufficient strength for supporting vehicular traffic and pedestrian traffic. Access covers which are subject to vehicular traffic must also be capable or resisting the horizontal forces imposed on the covers by motor vehicles, either during acceleration or during braking on the cover. These horizontal forces can be substantial. Square or rectangular covers inherently resist rotation when seated in a complementary shaped frame seat. However, if a square or rectangular cover becomes dislodged from the frame seat, there is a danger that the cover can fall through and into the hole, making retrieval of the cover difficult.
A round cover has the inherent advantage of not being able to fall through the circular aperture in the frame underneath. The disadvantage of a round cover is that the round shape does not inherently prevent rotation of the cover within the frame member. Consequently, an anti-rotation feature, i.e locking device, must be introduced into the cover and frame assembly. Known locking devices require precise rotational alignment of the round cover with respect to the frame member. Larger covers are difficult to maneuver and precise alignment to properly seat the cover is difficult to achieve.
Efforts have been made to lighten man hole covers by replacing steel and iron covers with composite covers that have skins of glass reinforced plastic material and a poly-urethane interior. Such covers are described in my U.S. Pat. No. 4,662,777 and such molding techniques taught therein are incorporated by reference. Man hole covers made of composite materials may still have significant weight and size that makes maneuverability difficult.
What is needed is an access cover and frame assembly that provides for a round cover to be automatically rotationally aligned as the cover is slid into place over the frame member.